This invention relates to a method of determining homogeneity of the contents of a number of bottles filled on a production line of control materials used in clinical chemistry.
Control materials which simulate analytical samples are essential for the accurate and reliable performance of many clinical tests. These control materials are usually complex mixtures and are commonly derived, in part, from natural sources. They are desirably prepared as a collection of identical samples and are expected to be stable over a designated period of time.
Clinical chemistry control materials are designed for use in analytical systems and in quality control programs to provide means for estimating precision and detecting deviations which result from reagent or instrumentation defects. These control materials also are useful for proficiency testing and interlaboratory surveys. If constituent levels are established with sufficient accuracy and precision, they may also be used as calibration standards.
Illustrative of such clinical chemistry control materials are those described in U.S. Pat. Nos. 3,466,249; 3,558,522; 3,629,142; 3,682,835; 3,705,110; 3,728,226; 3,729,427; and 3,753,925.
Recognizing the need for effective control materials used in the performance of clinical laboratory testing, the National Committee for Clinical Laboratory Standards has recently proposed that certain standards be set up for control materials sold in the United States; Clinical Chemistry, Vol. 18 , pp. 585-8 (1972). Among the criteria established by the NCCLS is that in the preparation of batches of controls, the manufacturer must ensure that there is a sufficient degree of vial-to-vial uniformity in the product so that the user may achieve reproducible results. One prerequisite of this uniformity is the homogeneity of the material being delivered into individual containers. The importance of homogeneity is readily apparent when it is considered that a control serum may contain from a few up to 30 different constituents. These constituents normally are admixed in a large container such as 1000 or 2000 liter kettle and then dispensed into unit vials of various sizes. According to the standards proposed by the National Committee for Clinical Laboratory Standards, inter-vial differences in concentration in the final product due to combined errors of homogeneity and dispensing shall not exceed .+-. 1.0 percent of the mean value in more than 5 percent of the vials. This represents a coefficient of variation (assuming a normal distribution) of 0.5 percent.
It is difficult to measure homogeneity in a batch of control liquid because the best biochemical test heretofore available is the assay system for sodium. Sodium has a coefficient of variation of about 11/2 to 2 percent in its assay procedures with a flame photometer. This assay requies a dilution of 1:200 and numerous errors can occur in this dilution series, as well as in readings on the instrumentation. In seeking a desired small coefficient of variation on the order of 0.1 to 0.5 percent for resolving power, this chemical approach is completely unfeasible for use in evaluating homogeneity of control serum, as adequate resolving power is not present in the method to be used statistically.
Physical methods also have been tried in an attempt to measure homogeneity of control serum, including use of an instrument to measure density of solutions to six significant places. The instrument must be held to within 0.01.degree.C., while making the measurements with a constant temperature thermostated system, and each measurement takes approximately 15 minutes to make. Because of the length of time required to make density measurements, and the fact that in a control group there may be as many as 1000 vials at a time to measure, this procedure is unfeasible in production work for measurement of homogeneity of control serum. Moreover, the value obtained for the density of water must be subtracted from the total density value in order to obtain the density of the solids contained in the solution measured. When the values were subtracted for water, the density values obtained were slightly above the numbers for sodium. The sodium values obtained were approximately 150, or three figures in a very low range. In the density measurements, the values obtained after subtracting the density of water from the density of the control serum were in the range of 300. The resolving power of this density measurement system thus was not sufficient, since multiple measurements of the same serum did not yield coefficients of variation less than 1%.
A method employing the refractive index of the solutions has also been used to measure homogeneity of control serum, but this too has been found unsatisfactory, even with the use of a five-place refractometer. The problem with this method is that it requires subtraction of the density value for water in order to obtain the refractive index of the solids contained in the control serum. This system is not sufficiently reliable statistically to give a low coefficient of variation. The values obtained after the values of water were subtracted from the values obtained for the control serum yielded density values in the 500 to 600 value range, which is insufficient significant figures for the accuracy required. The system required precision temperature control within 0.01.degree.C. and each value took about 5 minutes to measure which was totally unfeasible for production work.